Epistemology & Modern Physics

Source: The Emergence of Logical Empiricism (1996) publ. Garland Publishing Inc. The whole of Schlick selection for series is reproduced here.

There is no longer any doubt nowadays, that theoretical philosophy
has standing only in close connection with the sciences, whether
it seeks in them a basis on which it attempts to build further,
or whether they form for it merely the subject-matter of its own
analyses, whereby it then makes individual inquiry into the first
principles of knowledge. This is very much the case if, as I
believe, philosophy can be nothing else whatever but the activity
whereby we clarify all our concepts. And it is also beyond doubt
that, of all the sciences, physics here stands at the forefront.
Physics, that is, occupies an exceptional position, because in
it two elements are united, which are only found separately in
the other sciences: in the first place its exactness, the quantitative
determinacy of its laws, whereby it differs from all other factual
sciences, more particularly the historical sciences; and secondly
the fact that it has as its subject-matter the real, and
in this respect differs from mathematics.

Even a person who did not follow Kant, in permitting only absolutely
certain, exact knowledge to count as knowledge at all, would yet
be convinced that at any rate it represents the high point of
knowledge, so that a philosophy which could do complete justice
to exact knowledge would thereby at the same time have solved
the entire problem of knowledge. But this it can do, however,
only if it deals not merely with strict knowledge, but at the
same time with knowledge of the real, since merely imagined
or contrived objects are of little interest to the philosopher;
it is the world of reality which yields him the major problems.

Hence the physical sciences are assured of having a unique significance
for philosophy, though this has not always been apparent in equal
measure to philosophers of different periods. After the making,
in our own day, of some attempts that were already methodically
defective, to couple the historical with the exact sciences from
a philosophical standpoint, the modern development of physics,
which has taken on a highly philosophical character, has brought
out the peculiar position of this science a great deal more clearly
than ever before. So clearly, indeed, that, given the present
state of research, some altogether crucial questions about the
mutual relationship of physics and epistemology can perhaps be
brought to a decision.

The most important of these questions seems to me this: when,
through alliance with exact empirical science, philosophy emancipates
itself from the speculative procedure, does it acquire a better
criterion of its own truth? Given a proposition of physics, we
know how its truth can in principle be established: it has to
be confirmed by experience. But the question of how we actually
recognise the truth of a philosophical system is so far from having
found a generally satisfying answer, that it has often been put
forward only for the purpose of deriding philosophy.

Today, however, after the insight gained into the thorough interpenetration of philosophy and the sciences, we can and must say of epistemology, at least, that the correct theory is that which prevails in course of the advance of physical research.

But this formulation of the criterion of truth is initially so
indeterminate and general, that we still need very accurate elucidations
in order to understand its meaning aright. And here it is contemporary
physics alone which provides us with the instances of cognition
that are needed to specify and explain matters in full.

Before examining individual cases, we shall ask in what sense
we can really expect beforehand to find epistemological statements
confirmed in physics. Can philosophy predict any experimental
finding of the empirical sciences? We certainly have no right
to assume this, for if so, philosophy would be dabbling in the
trade of physics, and nobody believes any more nowadays that physical
results can be obtained by purely philosophical methods. The
task of epistemology is not to predict what will be observed in
nature. It merely tells us beforehand how science will react,
if this or that is observed. What it prophesies, therefore, is
not the results of experiments, but the impact of experimental
results on the system of physics.

By far the most important limiting case of such statements occurs
when it lays down specific principles with the claim that science
will always adhere to them, whatever sort of observations
may be made. In short, epistemology makes statements about the
dependence of physics on, and in limiting cases its independence
of possible observations. The statements are correct
when, on the occurrence of these observations, physical science
actually takes the form predicted.

Here now is the weightiest example from modern physics. The epistemology
pursued by the great mathematicians of the 19th century (Gauss,
Riemann, Helmholtz) had maintained that a specific course of processes
in nature (a specific mode of behaviour on the part of light-rays
and measuring rods) was conceivable, on observation of which physics
would turn over to employing non-Euclidean geometries. This prediction,
as we know, has been most brilliantly confirmed by the general
theory of relativity, and the premises on whose basis this prophecy
was made, have thereby demonstrated their truth-value. But what
role did these premises play in the epistemology of the said mathematicians?
Do they form the inmost heart of their philosophy, determining
the character of the whole edifice of thought, or are they of
a less essential kind, so that they might perhaps equally find
a place in an altogether different theory of knowledge? This
question has to be answered in order to know in what degree and
what aspect modern physics is actually to be seen as confirmation
of that particular epistemology, which was notoriously that of
empiricism.

An important step towards deciding the matter is taken if we establish
whether, or in what degree, the opposite theory to empiricism,
that of Kantian apriorism, would be equally capable of validating
the principles of modern physics. This apriorism teaches, of course,
that natural science will always adhere to certain general principles,
whatever any given experimenter may happen to observe.
These principles are said to be synthetic, that is, not to express
mere tautologies, and they are also said to be a priori. The
latter has a double significance in the Kantian system. First,
that is, that they represent logical presuppositions of science,
so that without them we could erect no structure of connected
truths about nature at all; but secondly, too, that these principles
are self-evident for us, so that we simply cannot imagine
their invalidity, and hence that our ideational consciousness
is inexorably linked to them. Of these two aspects the so-called
logical interpretation of Kant (the Marburg school) emphasises
the first, while the psychological view stresses the second.
The conflict between the two attitudes is strange, since both
interpretations are quite indubitably combined together in Kant:
synthetic a priori propositions are for him both the
logically necessary presuppositions of science, and also imbued
with the psychological compulsion of self-evidence.

Now which, according to the doctrine of apriorism, are the basic
synthetic judgements of all science? For Kant, they include the
axioms of Euclidean geometry, of which, as we have just seen,
modern physics demonstrates that they are not a priori in
the first sense, after it had already been made clear before this
that they are not so in the second (psychological) sense. For
in that sense apriorism with regard to Euclidean geometry has
already been refuted by psychological considerations, which many
philosophers still seem to overlook.

In this one connection, concerning some (and hence not yet all)
geometrical axioms, modern physics therefore opts decisively in
favour of empiricism. But apriorism can take a variety of forms;
its principle is elastic, and does not have to be defended precisely
in Kant's version. It would be quite generally refuted only if
it turned out that science contains no synthetic a priori
propositions whatsoever. Anyone who maintains their existence
must of course be able to produce them. An apriorism that cannot
really enumerate a single synthetic a priori principle,
has thereby pronounced its own death-sentence. For this reason
I raised the question some years ago' , as to which judgements
about nature a modern apriorisrn would now be able to propose,
in the light of contemporary physics, as absolutely inescapable
presuppositions of all science, independent of any possible observations.

And to this question modern scientific research appears to give
an answer of the same kind as that given in the case of Euclidean
geometry; for it shows that physical science refuses to
regard any one of the principles which might come into question
here as the sole possible basis. In order to convince ourselves
of this, let us go through the particular proposals which have
been made for keeping apriorism on its feet!

In the first place, now that a portion of the Euclidean axioms
has had to be dropped, the attempt has been made to extract a
complex from the remaining axioms of geometry, and to proclaim
it as the unshakeable foundation of all scientific accounts of
space. Reverting to an older belief, it has been sought to ascribe
this rank to the axioms of analysis situs, to those principles,
that is, which describe the purely qualitative inter-relationships
of space, without reference to 'metrical' relations of magnitude
- in short, to the axioms of 'topological' space'. But there
are indications in modern physics that it has no wish to allow
itself to be fettered for ever by such axioms. Hermann Weyl has
already outlined a peculiar theory of matter according to which
electrons, the ultimate constituents of matter, are as it were
outside space. The latter would have such peculiar topological
properties that it would be impossible, for example, to imagine
a spherical volume of space containing electrons to contract by
steady shrinkage into a point. Still bolder constructions are
scientifically possible, and there is simply no predicting the
assumptions to which we may be driven by the astonishing physical
facts disclosed by modern research. Hence the appearance of contemporary
physics gives us clear warning against the attempt to view the
topological axioms, say, as a noli me tangere [touch me
not].

In the second place, the language of the new physics pronounces
more clearly still against the endeavour to cling, say, to the
continuity of nature as a necessary and invariably satisfied
condition, which now finds expression in certain synthetic a
priori propositions. For since Riemann, some decades
ago, examined the physical possibility of a discontinuous space
composed of discrete points, Planck's quantum theory, in our own
day, has so domesticated the idea of jumpiness and discontinuity
in our view of nature, that our physics is nowhere prepared to
contest in principle the possibility of discontinuities.
Here too, therefore, apriorism finds no resting-place.

Third and lastly, let us examine the attitude of present-day physics
to that principle which appears in Kant as the most important
of synthetic a priori propositions, and is also
not infrequently declared to be such even today: I mean, of course,
the causal principle. If, appropriately enough, we mean
by causality the existence of regularity in nature, it certainly
represents a necessary presupposition of science; without causality,
a knowledge of nature would be impossible, for such knowledge
consists, in fact, of discovering laws. From this simple fact
many have already sought to conclude that the causal law is to
be regarded as an a priori principle in the fullest sense.
But this is undoubtedly quite mistaken, or at least a misuse
of terminology. For this does nothing to establish an epistemological
apriorism. The latter only comes about if we add the claim that
we should continue to uphold the validity of the causal principle
for all natural processes, whatever science may disclose to us
in. the way of facts in nature. In other words, we should have
to possess an unshakeable conviction of the factual validity of
the causal principle. We see here how the logical a priori
is inseparable from the psychological, if it is to characterise
a particular epistemological position, namely the Kantian notion
that our understanding prescribes laws to nature. So when Ernst
Cassirer expresses the opinion that the idea of universal regularity
in nature, as such, continues to hold good as a synthetic a
priori principle, or when J. Winternitz , among others,
describes the causal law as a constitutive principle of science
in Kant's sense, the view of these exponents of a modified apriorism
can only be understood to mean that they regard the possibility
of science as absolutely assured, and consider a nature that would
furnish no laws to man an absurdity.

As against this, it can be read off from the present state of
physics, that science does not recognise a priori constraints
of this kind, and opposes to the view in question the healthy
scepticism of the empiricist. The pursuit of processes within
the atom by the methods of quantum theory has led many physicists
to conclude that, within certain limits, processes that are strictly
causeless occur there; to these, therefore, the causal law could
find no applications.

Even if - like the author - one fails to perceive in the facts
available any sufficient basis for this conclusion, it could still
become perfectly legitimate if further facts were to hand, and
so this case has the following lesson to teach: Although physics
is well aware that the causal principle, the reciprocal dependence
of natural processes on each other, is a presupposition for its
own existence, it still by no means assumes this presupposition
to be satisfied a priori everywhere, — or even in a particular
area; it ascertains for itself, rather, using its own methods
(and with the exactitude of these methods), whether and to what
extent this is the case. It establishes for itself, that is,
the boundaries of its own kingdom. That the methods of science
are able to make such an examination, can be confirmed by a subsequent
analysis of its procedure. All this in contradiction to apriorism,
according to which the causal principle is supposed not to be
an empirically testable proposition.

The empiricist, of course, is well aware that it would always
be possible in principle to sustain the causal law by suitable
hypotheses -just as he knows that Euclidean geometry could be
held valid without exception, if we really wanted this; but he
denies that the human mind is unconditionally obliged to do this,
and denies also that the application of scientific methods could
always lead only to a confirmation of the causal principle. On
the contrary, it is quite easy to imagine observations which would
make it possible to sustain the causal law only by an infraction
of these methods: namely, by a continual introduction of new hypotheses
constructed ad hoc. And the modern physicist confirms
the empiricist's prediction the moment he thinks himself actually
confronted with observations of that kind.

Thus a survey of the state of modern physics indicates that it
presents us in surprising sequence with a series of cases, in
which the empiricist and apriorist views of natural knowledge
may contend with one another; that without exception it pursues
the course recommended by empiricism; and that not one of its
principles is accorded those properties which a synthetic a
priori judgement of the Kantian type would have to possess.

We may say, therefore, that modern physics shows us, that even
for epistemology there is a sort of confirmation by experience,
an objective criterion of truth, and that this criterion decides
in favour of the empiricist theory of knowledge.

A remark needs to be added, to guard against erroneous conclusions
from what we have said.

The relation outlined between modern physics and philosophy could
occasion regret that epistemology should cast the anchor of its
criterion of truth into empirical science, and thereby partake
of its uncertainty and mutability. But if the hope of grounding
philosophy on a firmer soil than that of experience and logic
must be abandoned (and it has never been more than a hope anyway),
this would have to be set off in the bargain against the advantage
of having obtained any objective criterion at all. It is very
notable that an actual exponent of apriorism, Elsbach (in his
book Kant undEinstein ), expresses the view that
epistemology can be expected only to vindicate the mutable state
of science at any time, but not science as such. This position
is no longer that of Kantianism (Einstein, in his critique of
Elsbach's book, says of him that he is in agreement neither with
Mohammed nor with the prophet); it is more empiricist than empiricism.
For the empiricist is unable to join in the lament of many onlookers,
that physics is constantly changing, that its theories are short-lived
and that hitherto supposedly correct laws are liable to be overthrown
at any moment by new discoveries. He knows, rather, that no law
till now, in the sense and with theexactitude whereby
it has once been confirmed, has ever again had to be abandoned.
The changeable elements in physics are not the relations of dependency,
which once established, continue to find repeated confirmation,
but rather the intuitive ideas which serve for interpretation
and interpolation. The split between the purely conceptual and
empirically confirmed content of a science, and the intuitive
images which illustrate the content without themselves belonging
thereto - this split is one of the most important achievements
of modern epistemology. A philosophy that knows how to achieve
it tidily everywhere may justifiably regard a confirmation by
modern physics in the sense outlined above as a confirmation by
science as such.